The present invention relates to a self-adaptive equalizer circuit for an installation which performs differentially coherent demodulation, more commonly called "differential demodulation". It is particularly applicable to the digital transmission of messages via a multipath channel whose transmittance is unknown a priori, thereby causing intersymbol interference to arise in the received message, e.g. digital transmission by radio beams using a tropospheric channel. The invention is applicable to installations that use linear modulation that can be demodulated differentially, such as binary phase shift keying (BPSK) and quarternary phase shift keying (QPSK), and it is advantageously applicable to receivers capable of operating in diversity mode.
The presence of intersymbol interference tends to degrade the performance of the receiver used in a digital transmission by radio beam very considerably. To remedy this effect, when using coherent modulation, it is known that the technique of self-adaptive equalization can be used by placing a self-adaptive filter after the demodulator.
Self-adaptive equalization is a known technique which consists in causing the received signals to pass through a self-adaptive filter whose function is to reduce intersymbol interference as much as possible. In conventional devices, this filter has a finite impulse response for so-called "linear" equalizer. It comprises two portions in "decision feedback" equalizers: a portion which acts on the received signal and another portion which acts on the symbols decided upon. In the systems that are commonest because they are the simplest, the filter optimization criterion is minimizing mean square error, and the filter coefficients are updated by the stochastic gradient algorithm.
With coherent demodulation, the algorithm for adapting the coefficients is well known, and numerous systems have already been made.
The drawback with coherent demodulation is the need to estimate the phase of the received signal. This is particularly difficult to estimate since the channels under consideration give rise to rapid phase variations that can cause the receiver to loose phase lock.
One way of avoiding the need to estimate phase is to use differentially coherent demodulation, more commonly called "differential demodulation". In this technique, the preceding sample serves as a "noisy" phase reference for the signal sample currently being processed. This simplifies receivers, but at the cost of a theoretical degradation in error rate performance.
As with coherent demodulation, the performance of differential demodulation is degraded when the transmission channel is not perfect. Past tests with self-adaptive equalization have been unsuccessful, firstly because it is not possible to place a self-adaptive filter at the outlet from a differential demodulator, and secondly because the known algorithms for updating filter coefficients do not apply to a circuit for differential demodulation.